The only way to calculate if you are using an extension cord of the right gauge is to do a little math. The total current draw, in watts, of everything that you are plugging into an extension cord is the number you need to make this calculation.
The easiest way to understand this is to use the 100 watt light bulb for an example.
A 14 gauge extension cord is rated to handle 15 amps. Assuming a nominal voltage of 120 volts (it can vary a little), that cable is rated to handle eighteeen 100 watt light bulbs.
Since you need to be able to translate this calculation to the actual devices you are using, you need to look at your devices (tools, lights, etc.) as there should be a power consumption rating, in watts, right on the device.